Intermolecular distance and density scaling of dynamics in molecular liquids

A broad variety of liquids conform to density scaling: relaxation times can be expressed as a function of the ratio of temperature to density, the latter raised to a material constant γ. For atomic liquids interacting only through simple pair potentials, the exponent γ is very nearly equal to n/3, where n is the steepness of the intermolecular potential, while for molecular liquids having rigid bonds and built using the same interatomic potential, γ > n/3. We find that for this class of molecular liquids, γ = n/δ, where the parameter δ relates the intermolecular distance to the density along an isomorph (the line of approximately constant dynamics and structure). δ depends only on the molecular structure and not the interatomic potential.